The Explosion of Applications in Fiber Optics since 1960

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The Explosion of Applications in Fiber Optics since 1960

Overview

Used in communication, fiber optics is the technique of sending light waves through glass fibers. It is analogous to communicating via radio waves in that signals are encoded at one end for transmission and decoded at the other. Fiber optics was developed sporadically throughout the first half of the twentieth century. Since the 1960s the field has exploded as material sciences and computer technology have made fiber optics more efficient. Today around 80% of long-distance communication is carried through fiber optic networks.

Background

Long-distance communications via optical signals has been with humanity for many centuries, ranging from smoke signals to sailing ships communicating with flags. The first mechanical optical communication systems were semaphore towers, developed in France in the 1790s.

During the nineteenth century many of these systems were made obsolete by telegraphy and the telephone. Alexander Graham Bell, the inventor of the telephone, also developed the Photophone, which communicated with light instead of electrical signals. The invention didn't get very far since light doesn't travel well through the air: a single building, cloud, or hill can effectively block any signal.

In the 1840s it was shown that light could be guided along jets of water. This was used to create fantastic, elaborate fountain displays. These scientists took advantage of the phenomenon called total internal reflection. Because water has a higher refractive index (a measure of the ability to bend light waves) than that of air, light beams will remain inside the water. This same principle was taken advantage of by dentists who used bent quartz rods to illuminate the mouth.

The first true demonstration of image transmission through glass was made by the German medical student Heinrich Lamm in 1930. He was able to transmit the image of a light bulb filament, but the quality was very poor.

The first transmissions of images through bundles of tiny drawn glass fibers were made independently in 1954 by Abraham van Heel, and the team of Harold Hopkins and Narinder Kapany. This was the discovery that broke open the field of fiber optics.

Impact

It was about 20 years from the invention of working bundles of fibers to practical communication systems. The biggest problem with the first fiber optic systems was signal strength. Signals sent through glass tend to attenuate, or die off, leaving the receiver on the end with a fuzzy, indecipherable signal. This was because the glass fibers were contaminated and light sources were not powerful or easily controllable. Two factors were indispensable in overcoming this problem: the laser provided a focused, powerful signal and new techniques and materials were used to make higher quality, pure, coated glass fibers.

The laser was the biggest step forward in providing a powerful, precise light source. It was invented in 1960. It provided a focused, coherent beam of light with an exact wavelength. Now scientists could tune a light wave transmission just as precisely as a radio transmission. Too, the shorter wavelengths of light beams theoretically allowed scientists to place much more information into a signal. But the first lasers were impractical for commercial purposes: they had to be cooled to almost -200 degrees C, and even then worked for very short periods, just seconds or minutes, before they burned out. Lasers that worked at room temperature weren't developed until 1970, and durable lasers with a lifetime of 10 years weren't made until 1976.

The other problem, signal loss, was overcome in stages. Glass fibers prior to the 1950s were not very pure, had optical aberrations and "leaked" the signal due to the poor quality or absence of coating materials. The first fix was the introduction of cladding, a coating that surrounded the glass fibers. Many researchers tried applying a plastic coating, but these had little effect on the optical quality. In 1956 Lawrence Curtiss, at that time still an undergraduate student at the University of Michigan, successfully coated glass fibers with a cladding made of a different glass. This cladding had a lower refractive index than that of air, which prevented loss of the signal into the air as well as the introduction of extraneous signals from the air. It also insulated each fiber from its neighbor, virtually eliminating cross-talk between fibers.

The last key transformation was a viewpoint shift as well as a technological shift. Many researchers believed that the glass itself was responsible for the rapid degradation of the signal. No matter how pure they made the glass, it would still degrade the signal. Things began to change when Charles Kao, a scientist at Standard Telecommunication Laboratories believed that if the purity of the glass was increased it would conduct light almost perfectly. His continual pushing for better fibers began a competition between many research centers.

The winner of this race was Corning Glass Works. In 1970, Robert Maurer, Donald Keck, and Peter Schultz invented a way of preparing fused silica (the glass) in an almost pure form by controlling how the fibers were drawn, the temperature needed to form the glass, and what chemicals were added. Their glass had a slightly higher refractive index than its cladding because the chemical titania (titanium oxide) was slowly doped (added) into the silica. Proper control of the amount of titania kept the refractive index high and the signal inside the glass protected by the cladding. Although the resulting fibers were fragile they had a far lower signal loss than any competing fiber. Not long after the Corning scientists found another way of improving the flexibility of the fibers by doping them with germania (germanium oxide).

The first practical fiber optic communication system was put into place in September 1975 in Dorset, England. The police in that area were looking for a new system after a current surge from a lightning strike destroyed the station's electronic systems. The police chief wanted a new system that didn't depend on electrical wires. Since fiber optic systems use light instead of electricity they are less susceptible to power surges. The English system took only a few weeks to set up and worked very well.

AT&T tested its first fiber optic system in Chicago in 1976. They linked three office buildings in the downtown core with 2.6 kilometers (1.6 miles) of optic cable. They began tests on April 1, but were beaten to the punch by GTE, which was able to get 10 kilometers (6.2 miles) of fiber optic cable up and running for public use in Long Beach, California, on April 22. GTE's lines couldn't carry as much information as AT&T's, but it was the first working American system.

After these initial successes the first large network to be installed was the Boston-New York-Washington corridor. The previous network was composed of old microwave transmitters and carried more telephone calls and communication than almost anywhere else in the world, due to the large concentrations of industry, government, and finance companies along the American coastline. AT&T chose to line it with fiber optic cables in 1980. The line was finished in 1984, but by then changes in fiber optic transmission technology had moved so quickly that it was almost obsolete. MCI had started putting in a better system using different technology standards in 1982 with faster data transfer, more bandwidth, and fewer repeating stations. (Repeating stations amplify fading signals for transfer along the next section of cable.)

The other big goal of fiber optic communications was to stretch cables across the sea, joining the continents. The first submarine telegraph cables were laid across the Atlantic in the 1850s. None of these cables lasted very long—the first worked for 28 days—but the communications sent during those periods were valuable enough that the entrepreneurs of the time kept developing their systems.

By the 1960s there were two ways of communicating between continents with a telephone. The signal could be sent via submarine cables or through satellites. Cable transmissions tended to be static-filled, weak signals while satellite transmissions were marred by the time delay it took radio signals to travel up to orbiting satellites and back down again, plus other problems like repeating echoes, feedback, and static. Either way was extremely expensive.

The biggest problem in laying long-distance cables is that signals need to be amplified often: the TAT-6 coaxial cable, which uses electrical signals and was laid in 1976 under the Atlantic carries almost 1,000 repeaters, or about one every 9.2 kilometers (5.7 miles). These are also the parts most likely to break. Any break is extremely expensive to repair, since a ship with technicians must be dispatched to find the break, haul the cable up from the seafloor, and repair it. Therefore any way that the number of repeaters can be reduced is extremely valuable. As well, coaxial cables were approaching fundamental size limits. Anything bigger would be overly susceptible to breakage and too large to fit on cable-laying ships.

In 1978 the first fiber optic submarine cable across the Atlantic was proposed. It would be called TAT-8 and would carry fewer repeaters and more voice channels than any of the earlier coaxial cables. It was completed in 1988. Many more have been laid since then, and today 80% of worldwide long-distance communication is carried through 25 million kilometers (15,534,279 miles) of fiber optic cables.

The last problem facing fiber optics is connecting them to homes. Today fiber optic cables circle the world, delivering communications to every continent. But the final place they don't reach is one of the most common: homes. Fiber optic telephone signals reach a "neighborhood" where they stop at a hub. Here the signals are converted for transmission to groups of 100-2,000 homes via coaxial cables. This switchover slows down signals that could travel much faster both in and out of homes were the signal carried entirely through fiber optic systems. The switchover has not been made because installing fiber optic systems still costs more than coaxial cables, even though the price continues to fall. The expense isn't with the fibers themselves, but with the costs of switching—including paying skilled technicians, and installing adapters for televisions that aren't equipped to receive fiber optic signals. Still, some observers think that since the changeover must come eventually, sooner is better than later, especially with the growing use of the Internet and the increase of telecommuting workers. In Italy, the government-owned Telecom Italia has already begun this conversion process and, except for a few experimental communities scattered throughout the world, is far ahead of the rest of the world.

PHILIP DOWNEY